The invention is generally directed to an electronic printer and in particular to various subsystems of a printer including the power transmission, clutch initialization system, head movement and ribbon movement mechanisms.
Although a variety of mechanisms have been used to transmit or interrupt motive power utilizing clutches most utilize a plurality of clutches where each clutch is controlled by a corresponding electromagnetic member. The electromagnetic members have the largest number of parts, are most expensive and occupy the largest space among the components utilized in a printer. As a result, in multiclutch power transmissions for printers, bulkiness and expense have proved troublesome, particularly in the development of portable and inexpensive printers. Accordingly, there is a need for a power transmission for a printer which reduces the need for an electromagnetic member for each clutch. It is specifically desired to provide a power transmission for a printer which controls power transmission with two clutches but only one electromagnetic member by making use of both the forward and backward rotation of the motor.
The invention is also generally directed to a power transmission apparatus in which the printer can be initialized in a desired standby position by using a clutch which can restrain the rotating angle of the clutch to less than one full rotation. In printers using a clutch able to set the rotating angle at less than one full rotation it has been usual to connect a detector at the output side of the clutch corresponding to the divisions of the perimeter of the clutch. For example, a cam is connected to show the corresponding position of the clutch. However, when the printer or other apparatus is started it is necessary to check the stopped position (i.e., phase on the output side) of the clutch which has been divided into several parts. As a result, a detector provided on the output side of the apparatus to detect the phase of the stopping position of the clutch was required. The apparatus therefore has an increased size and cost, because of the additional parts required. Also, the reliability of the detector varies depending upon its complexity and design. The need for a detector to establish the stopped position of the clutch is inconvenient in an apparatus such as a portable and inexpensive printer. Accordingly, there is a need for a clutch initalization mechanism which eliminates the need for a detector and which places the clutch in a desired standby position without the need to first check the positioning of the clutch.
The invention is also directed to a carriage movement mechanism for a printer which moves the carriage by the engagement of a carriage shifting cam on the carriage with a plurality of shifting pins along the range of movement of a carriage. Various arrangements of shifting cam members and shifting pins have been used to shift the printer carriage. However, a problem with these devices is the locking of the shifting cam and attached shifting shaft at both ends of the movable range of the carriage. This results in unacceptable forces being applied to the mechanism.
As an attempt to overcome this problem another approach has been tried which provides a region between the point at which the carriage stops and the position at which it locks. A second proposed solution was to remove the pins on both ends of the movable range to disengage the shifting cam from the pins on the ends. In these situations the whole carriage is pushed to an engaged side with a pin by a spring or other biasing device to re-engage. However, the rotation of the shifting shaft to which the shifting cam is coupled cannot be utilized for other operations. In the first attempted solution the total number of revolutions of the shifting shaft is limited by the range between the end of the pins and the locking position. In the second attempt to solve the problem a large spring is necessary to move the entire carriage which increases the vibration and noise to a level that is unacceptable for use in a low-noise printer such as a thermal printer. Accordingly, a carriage movement mechanism which allows for the continuous rotation of the shifting shaft with low vibration and noise is desired.
The invention is also directed to a thermal print ribbon movement mechanism which prevents the advance of the thermal print ribbon when no characters are printed. Ribbon take-up mechanisms for thermal printers using thermal ribbons and in particular thermal ribbons mounted on a carriage have had take-up of the ribbon controlled by a take-up motor or other power device. In other approaches the take-up mechanism achieves its power by converting a force in the direction of carriage movement into a turning force by the combination of a rack and pinion, friction plate and friction wheel. The take-up of the thermal ribbon is controlled by adjusting the engagement between the rack and pinion or friction plate and friction wheel by rotation of the entire carriage. The first approach requires a motor or other power source exclusively devoted to the take-up of the ribbon, which is expensive and occupies significant amounts of space. In the second approach a large force is required to rotate the entire carriage and a complicated mechanism unavoidably results. In addition, the printer dimensions must be enlarged so that the carriage is free to rotate within the case. Accordingly, there is a need for a thermal print ribbon movement mechanism which does not require the need for an additional power source or require the rotation of the carriage.